Internal validity is a form of experimental validity (1). An experiment is said to possess internal validity if it properly demonstrates a causal relation between two variables (2,3). An experiment can demonstrate a causal relation by satisfying three criteria:

That the "cause" precedes the "effect" in time (temporal precedence),

that the "cause" and the "effect" are related (covariation), and

that there are no plausible alternative explanations for the observed covariation (nonspuriousness) (4).

In scientific experiments, researchers often manipulate a variable (the independent variable) to see what effect it has on a second variable (the dependent variable) (5). For example, a researcher might manipulate the dosage of a particular drug to see what effect it has on a person's health. If the experiment allows the researcher to conclude that different doses of the drug caused a change in peoples' health (by satisfying the above criteria), then the study possesses internal validity.

In other words, an experiment possesses internal validity if the observed changes in the dependent variable were caused by the manipulation of the independent variable.